Soul Tithe | Magic: The Gathering
Formats Soul Tithe is Legal in
Official Oracle Text for Soul Tithe
At the beginning of the upkeep of enchanted permanent's controller, that player sacrifices it unless he or she pays , where X is its converted mana cost.
Official Rulings for Soul Tithe
10/1/2012 : If the permanent has in its mana cost, X is 0.